What they knew was chilling: The men were not government soldiers at all. They were members of the ruthless Islamic extremist group called Boko Haram. They kidnapped the entire group of girls and drove them away in pickup trucks into the dense forest.

Three weeks later, 276 girls are still missing. At least two have died of snakebite, and about 20 others are ill, according to an intermediary who is in touch with their captors.

Their plight — and the failure of the Nigerian military to find them — has drawn international attention to an escalating Islamic extremist insurrection that has killed more than 1,500 so far this year. Boko Haram, the name means “Western education is sinful,” has in a video seen Monday claimed responsibility for the mass kidnapping and threatened to sell the girls. The British and U.S. governments have issued statements of concern over the fate of the missing students, and protests have erupted in major Nigerian cities and in New York.

The 16-year-old was among about 50 students who escaped on that fateful day, and she spoke for the first time in a telephone interview with The Associated Press. The AP also interviewed about 30 others, including Nigerian government and Borno state officials, school officials, six relatives of the missing girls, civil society leaders and politicians in northeast Nigeria and soldiers in the war zone. Many spoke on condition of anonymity, fearing that giving their names would also reveal the girls’ identities and subject them to possible stigmatization in this conservative society.

The Chibok girls school is in the remote and sparsely populated northeast region of Nigeria, a country of 170 million with a growing chasm between a north dominated by Muslims and a south by Christians. Like all schools in Borno state, Chibok, an elite academy of both Muslim and Christian girls, had been closed because of increasingly deadly attacks by Boko Haram. But it had reopened to allow final-year students to take exams.

At about 11 p.m. on April 14, a local government official, Bana Lawal, received a warning via cell phone. He was told that about 200 heavily armed militants in 20 pickup trucks and more than 30 motorcycles were headed toward his town.

Lawal alerted the 15 soldiers guarding Chibok, he said. Then he roused sleeping residents and told them to flee into the bush and the nearby hills. The soldiers sent an SOS to the nearest barracks, about 30 miles (48 kilometers) away, an hour’s drive on a dirt road.

No help arrived.

When the militants showed up two hours after the warning, the soldiers fought valiantly, Lawal said. Although they were outnumbered and outgunned, they held off the insurgents for an hour and a half, desperately waiting for reinforcements. One was killed. They ran out of ammunition and fled for their lives.

As dawn approached, the extremists headed for the boarding school.

There were too many gunmen to count, said the girl who escaped. So, even after the students realized the men were Islamic extremists, they obediently sat in the dirt. The men set the school ablaze and herded the girl’s group onto the backs of three pickup trucks.